I took a look at some charts on metals, for thermal conductivity and density.
Pure copper is the best, about twice as good as the second best (non exotic/precious) metal, which is aluminum. It is also over 3x as dense, so that would make it potentially 6x better for the same thickness of check/jacket.
But I don't think gas checks are pure copper? Not sure what alloy is used in gas checks, but the thermal conductivity can go way down for alloys. Eg. brass is 70% copper and 30% zinc, and it has only half the thermal conductivity as aluminum (but since it is 3.5x as dense, it's still nearly twice as conductive for a similar thickness of check). You want to burn your fingers, put a piece of brass against a disc sander with your bare hands.
Zinc washers have also been used as "gas checks." This is the reason I looked up these charts. I already knew copper and aluminum as the two metals used in heat sinks. Surprise surprise. Zinc is also a very good thermal conductor, a bit better even than brass and nearly as dense.
This thermal conductivity factor may have been overlooked, to date, by us reloaders. This might be part of the reason gas checks are generally thicker as the caliber increases. A 223 check doesn't have very far to spread the heat to.
Lead has a very low thermal conductivity down there with steel. I.e., where the lead bullet is getting heated, you can more easily melt the surface off while the core is still cold. This is one of the reasons it is so easy to weld steel. Aluminum has a way lower melting point, but it's more difficult to weld, partly because it's harder to control the spot that melts, even after preheating the entire piece to avoid that initial heat sinking/spreading.
Soda can checks might work fine to reduce/eliminate fouling. But I wonder if they retain accuracy when pushed hard enough, compared to thicker checks. Beside being really thin, they also have a thin plastic coating on the inside and some sort of coating or at least paint on the outside. I suppose that wouldn't be too different from the oxide layer that will be on normal checks. But I would suspect soda can checks might work a little better if you make them inside out, though a bit less colorful.
Defense industry, are you seeing this? To improve tank armor against shaped charges, maybe we ought to be laminating the steel armor with layers of high temperature thermal conductors. (Probably already doing that.)
Mythical silver bullets: silver has a fantastic thermal conductivity, almost as good as pure copper and even more dense. It might make a good material for a jacket or a gas check. Gold is nearly as good, its way denser than even lead, and the melting temp approaches steel. Might make for a good jacket or even solid super high velocity bullet. Should be relatively affordable if the price of primers keeps going up.
I'm not sure why this post would make anyone angry, but I'm sure it will.
If you are shooting normal cast velocities, using the right speed of powder, the correct alloy and quench and PC/GC, no your bullet won't melt. But the max velocity and pressures of your higher velocity rifle cartridges will be much lower than for jacketed, per the reloading manuals. There seems to be a cap on pressure and velocity, for all practical purposes, just looking at Lyman cast data.
When my bullets were spraying out the muzzle, I am pretty sure it was because my bullets were too soft, because I was using a powder and charge that was commonly recommended. Most everyone suggests to water drop, in order to get cast 223 up to at least 20 BHN. I used a mix of 1/3 lino, but I didn't do the water quench. I managed to get my rifle to shoot and cycle, with a lesser load, but that clogged up my gas port with lead. I have read that cast bullets clogging a gas port is a complete myth, just like melting of bullets.
|
   
   
|
|
Reply With Quote
